In evaluating testimony, the judge, after having requested testimonial letters if necessary, is to consider the following: 1/ what the condition or reputation of the person is; 2/ whether the testimony derives from personal knowledge, especially from what has been seen or heard personally, or whether from opinion, rumor, or hearsay; 3/ whether the witness is reliable and firmly consistent or inconsistent, uncertain, or vacillating; 4/ whether the witness has co-witnesses to the testimony or is supported or not by other elements of proof.
Source: Code of Canon Law (Vatican.va)